1. Field of the Invention The present invention relates to a projection display device that modulates light on the basis of image information to form an optical image, and projects the formed optical image.
2. Description of the Related Art
A projection display device includes at least a light source, and an optical modulator that modulates light emitted from the light source to form an optical image. As a general light source of a projection display device, a discharge lamp such as a metal halide lamp, a xenon lamp, or a high-pressure mercury lamp is used. In recent years, a projection display device that uses a light emitting element such as a light emitting diode or a semiconductor laser as a light source has also been developed. Meanwhile, as a general optical modulator of a projection display device, a liquid crystal panel or a DMD (Digital Micro-mirror Device) is used.
To increase brightness of a projected optical image, an optical modulator needs to be illuminated by a brighter light. For the optical modulator to be illuminated by a brighter light, an power of a light source needs to be increased. To increase the power of the light source, when the light source is a discharge lamp, electric power input to the discharge lamp needs to be increased. Also, when the light source is a light emitting element, a drive current of the light emitting element needs to be increased. However, when the power of the light source is increased, a heating value is increased to overheat the light source, causing various problems. For example, the amount of light emitted by the light emitting element changes according to the operating temperature. Generally, the amount of light emitted by the light emitting element decreases with the increasing operating temperature. Also, when heat generated from the light source increases temperature in a casing of a projection display device, operation of an optical element or an electric element other than the light source becomes unstable or life of the element is shortened.
For a liquid crystal panel or a DMD that is an optical modulator, an operating temperature range is set. When an operating temperature exceeds a predetermined temperature range, operation of the liquid crystal panel or the DMD becomes unstable or impossible. When heat generated from the light source increases the temperature in the casing of the projection display device, the temperature of the optical modulator also increases, but the main heat source for increasing the temperature of the optical modulator is the light applied to the modulator. Specifically, a part of the light emitted from the light source and applied to the optical modulator is converted into heat, and the heat increases the temperature of the optical modulator. Thus, an increase in the power of the light source increases the temperature of the optical modulator.
Thus, various techniques for cooling a light source or for cooling an optical modulator have been developed. Japanese Patent Laid-Open No. 2005-74076 describes an example of a technique for cooling a light source. Japanese Patent Laid-Open No. 2004-354853 describes an example of a technique for cooling an optical modulator. Further, Japanese Patent Laid-Open No. 2006-154460 describes a projection display device including a temperature sensor, and a cooling fan controlled on the basis of the temperature measured by the temperature sensor.
In short, in the field of a projection display device, an efficient manner of cooling a light source and an optical modulator is one of the important subjects in development of the device.
However, with the widespread use of projection display devices, projection display devices have been used in various environments. Thus, the temperature of the optical modulator may become lower than the operating temperature range depending on environmental temperature. For example, when the projection display device is used outdoors in winter or at high altitudes, the temperature of the optical modulator may become lower than the operating temperature range.
When the temperature of the optical modulator becomes lower than the operating temperature range, operation becomes unstable or impossible such as when the temperature becomes higher than the operating temperature range.